1. Field of the Invention
The present invention relates to negative active material compositions for rechargeable lithium batteries, to negative electrodes for rechargeable lithium batteries, and to rechargeable lithium batteries including the same.
2. Description of the Related Art
Lithium rechargeable batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials. Lithium rechargeable batteries also use organic electrolytes or polymer electrolytes between the positive electrodes and the negative electrodes.
Lithium rechargeable batteries generate electrical energy from oxidation and reduction of lithium ions during intercalation and deintercalation at the positive and negative electrodes. Such rechargeable lithium batteries have high cell voltages and large energy densities due to the high standard electrochemical potential of lithium ions.
Lithium-transition element oxides have been used as the positive active material of rechargeable lithium batteries. Crystalline or amorphous carbon or carbon composites have been used as the negative active material.
Each of the positive and negative active materials is coated on a current collector at an appropriate thickness and length to fabricate positive and negative electrodes. Alternatively, each of the positive and negative active materials is formed into a film to fabricate positive and negative electrodes. The positive and negative electrodes and a separator positioned between the positive and negative electrodes are then wound or stacked to fabricate an electrode assembly. The electrode assembly is placed in a can or other case and an electrolyte solution is injected therein to fabricate a prismatic rechargeable battery.
Generally, the negative electrode for a rechargeable lithium battery is fabricated by mixing a negative active material, a binder, and optionally a conductive agent to prepare a negative active material composition, and coating the negative active material composition on a current collector to form a negative electrode.
Polyimide has good adhesion characteristics and shape stability at high temperatures, and therefore can be used as the binder for the negative electrode of a rechargeable lithium battery requiring stability at high temperatures. Polyimide is not used itself, but rather a precursor of polyimide is used in the negative active material composition. Polyamic acid (which is a precursor of polyimide) is converted into polyimide during drying of the negative electrode preparation. The conversion reaction, which is a dehydration reaction, causes shrinkage of the polyimide, resulting in bending of the negative electrode. In addition, the polyimide is rigid.